Cost-Effective Passive Solar Design

Orient your house properly, include an adequate amount of south-facing glazing protected by roof overhangs, and include some interior thermal mass

Plenty of south-facing glazing. One way to lower costs is to specify as many fixed windows as possible, since fixed windows cost less than operable windows. This passive solar home was designed by Springtime Homes of Asheville, North Carolina.
Image Credit: Brian Knight

Passive solar design is one of the most attractive strategies available for energy-efficient construction and green building. The sun provides free heat, daylighting, and a better connection to our outdoor environment. It does this for the life of the structure.

If you follow these priciples, your house will offer passive survivability, meaning it will remain livable through winter power outages. The passive elements of your home design will have no moving parts, and the only maintenance need is occasional window cleaning.

The best part: passive solar design can be done with zero extra upfront costs.

Start with a site with good solar exposure

Not every building site or lot is suitable for passive solar design. Strategies will vary depending on latitude, material selection, design, and site conditions. These methods are adjustable to fit one’s site and needs.

Just as shoppers need to be alert to the exaggerations of greenwashing, homeowners should be alert to the liberal use of the phrase “passive solar” by builders and designers. Just because a builder claims that a house has been designed according to passive solar principles, doesn’t mean it has.

Passive solar design requires stretching a building’s shape from east to west while prioritizing southern windows. Here are some important details of passive solar design, in order of importance (after assuring that the house has a tight, well insulated building envelope).

1. Calculate the area of south-facing glass as a percentage of the home’s conditioned floor area:

7% or less is considered “sun-tempered.”

9% to 12% is the ideal range that most passive solar designs aim for.

More than 12% puts the house at risk of overheating unless the design includes extra thermal mass. Other concerns with higher glazing ratios include increased costs, glare, and excessive thermal losses.

The higher a window’s solar heat-gain coefficient (SHGC), the better. Be aware of the difference between the glazing-only SHGC and the whole-window SHGC. A whole-window SHGC (the number on the NFRC label attached to the window) should be at least 0.40; a better target is 0.56. Check out the new Cardinal low E 180 glazing; it is a replacement for the Cardinal 179 glazing. The manufacturer rates this product (these are glazing-only specs) at U-0.26 with a SHGC of 0.69.

It’s possible to use a higher glass ratio with a lower SHGC and get similar results to a lower glass ratio with a higher SHGC, but this approach increases your material costs. To be sure you get the energy performance you expect, check your glazing plan using RESFEN or another type of energy modeling software.

3. Design an appropriate shape and orient the house correctly

Most designers agree that the more you stretch a building’s shape from east to west, the better it will perform on sunny days. This helps by providing more wall area for southern windows. It also minimizes the impact of intense east and west sun angles during the cooling season. There is a downside, however: such buildings lose more more heat at night than a cube-shaped building. Good energy-modeling software will help a designer optimize a building’s shape.

Perfect orientation is not always possible, since the building site often dictates the design. Facing within 20 degrees of true south is ideal, but some designers feel that up to 40 degrees is still advantageous. In a climate with cooling needs, it’s better for the house to face southeast than southwest.

Generally, passive solar design requires overhangs that are sized to fully shade the south-facing windows on the summer solstice (June 21) and to allow full sunlight to enter on the winter solstice (December 21). A strict implementation of this guideline is not necessary, but be aware that some state tax credit programs and green certification points may require it.

Correctly sized overhangs are especially important in climates where homes need air conditioning. Since the global climate is warming and the use of AC is dramatically increasing, it’s safer to include overhangs in most cases. Since they help protect siding from the weather, they provide maintenance benefits as well. In some designs, like the one pictured above, the overhangs cost us nothing extra.

Getting overhangs sized correctly seems to give designers a lot of trouble, especially on houses with multiple stories. Each floor level needs its own overhang. A rule of thumb that works for most U.S. latitudes is that overhangs should be 12 inches above the window and 18 inches deep. This is an easy variable to adjust depending on window height, latitude, orientation, and design situation.

The best Web-based tool to help designers of passive solar homes with overhang sizing is the Overhang Design Tool on the Sustainable By Design website. Sketchup is another good option.

5. Include thermal mass

How much thermal mass is required in a passive solar house? This is perhaps the most controversial element of passive solar design, and probably the least important. Yet it could easily be argued that thermal mass is more important than overhang design, especially the further north you go.

Most people feel that a designer who includes a lot of thermal mass can see the cost-effectiveness of the project drop dramatically. However, there is plenty of evidence showing that thermal mass inside the conditioned envelope can improve a building’s thermal performance. The question is, how much more are you is willing to spend in time, materials, and labor to install additional thermal mass to achieve the supposed benefits the mass provides? The question is debatable.

Uncarpeted concrete slabs make the most sense, since the concrete is a component that is often needed anyway for slab-on-grade and basement construction. Stained and polished concrete floors are increasingly recognized as one of the most aesthetic, low-maintenance, high-performance floor finishes available.

If you are designing a passive solar house for a building site that is ideal for a crawl space, consider building stem-walls filled with dirt or gravel and topped with a slab. A stem-wall foundation with a concrete slab is a common building practice that can cost less than a sealed crawl space. Such a foundation has more thermal mass and less space to condition. A slab is also (arguably) healthier than a crawl space.

Beware of advertised benefits from products that don’t have their thermal mass completely inside the insulated, conditioned space. We’re looking at you, ICFs and ACC (autoclaved aerated concrete). These can be appropriate products, but don’t expect much benefit from the thermal mass, unless you live in a high-elevation desert with wide diurnal temperature swings.

Some designers and builders go through a lot of extra expense and trouble to increase the amount of glazing, and then address overheating concerns with systems to actively or passively move and store heat in additional thermal mass. Lots of experimentation has been done using many methods and materials, with mixed results. These methods can work, but they add cost and complexity, and they can introduce serious problems. Windows and concrete slabs are simple and maintenance-free. They work extremely well and most designs need them anyway.

The most important detail of a passive solar design is an airtight, continuously insulated building envelope. Free heat does us little good if we can’t control where it goes. A building envelope is THE most important component of passive solar, energy efficient and high performance homes and buildings.

One of the ways that passive solar design can add zero extra upfront costs is by refining the window sizes and placement. By decreasing the windows on the non-south sides of the home, we can stay balanced in costs and can decrease year-round thermal losses while increasing needed solar gain.

Other tricks for achieving cost savings and improved performance:

Increase the number of large fixed windows. If you specify as many fixed windows as possible, the costs per square foot of window drops dramatically. You’ll also see an increase in energy performance because there is less thermal bridging and more solar gain. Fixed windows are easier and cheaper to equip with blinds and movable insulated curtains than operable windows. For the most part, fixed windows are easier to clean. However, there is a legitimate concern about the difficulty of cleaning fixed windows on upper levels. One good strategy is to place an operable casement nearby to facilitate cleaning from the inside. Be sure it opens the right way.

Decrease the number of operable windows. It’s common to see designs with far too many operable windows. They are expensive, have more air infiltration, require maintenance, decrease solar gain, and increase thermal bridging. One operable window per room is usually plenty for ventilation, especially if you are choosing casements. A single bedroom egress window is already plenty big. Locating windows opposite doors helps with ventilation and makes rooms feel bigger.

Avoid extra engineering. Another good reason for keeping your glass ratio in the 9% to 12% range is that one can create large areas of wall that are uninterrupted by window openings, increasing shear resistance. This can eliminate the extra expenses of engineering, materials, labor, and energy costs of needing structural members where your insulation should be. Solid wall area can help with interior furniture placement and is a good location for stairwells. This strategy also helps you avoid the use of pricier tempered glass.

Avoid muntins and dividers. They cost more, decrease solar gain, interrupt views from inside, and make it harder to clean the window if they aren’t between the panes.

What about trees?

In regards to deciduous shading on the south side of the house, the best practice is to remove trees if possible and never plant anything that could eventually grow to shade the south windows.

We have found that passive solar design can still work, even with deciduous shading. We recently completed two nearly identical passive solar homes using the same materials and achieving the same ACH50 of <1. The home that has considerable shading from deciduous trees performs nearly as well as the home with zero shading.

Should I plan for air conditioning?

Some builders in our area have built excellent passive solar designs without air conditioning or mechanical ventilation, hoping that the extra thermal mass of concrete floors on upper levels would save them during the cooling season. Having been in three of these homes during the summer, however, I can say that this strategy is not working very well.

Asheville is a mixed-humid climate (climate zone 7, near zone 6). Even with excellent passive ventilation practices, passive solar designs with lots of thermal mass should include a means of mechanical cooling in most U.S. climates.

Passive solar design matched with an airtight, building envelope with minimal thermal bridging offers builders and designers a cost-effective way of reducing heating costs by 40% to 90% while providing abundant daylighting and a better connection to the outdoors. Spread the word and share your opinion or experience on one of the most appropriate energy-efficient construction and green building techniques available.

Brian Knight is a builder and the owner of Springtime Homes in Asheville, North Carolina.

22 Comments

A place in the sun
What with passivhaus, net-zero and super-insulation stealing much of the limelight these days I'm glad to see the principles of good passive solar design thoroughly restated in this article. I hope and believe that an integration of all these approaches is the way forward in high performance home design. It does seem that some of the early attempts at hybrid passivhaus/passive solar designs are clumsy and confused - probably because of the similarity in names - but I believe we will get past this in time to a better way of building and a better understanding of what makes a truly green home.

Meanwhile my favorite definition of passive solar remains:
"A comfy place to take a nap in the sun'.

A different Contrarian view of overhangs
Your link to Peter Powell's post above is interesting and food for thought, and I would like to submit some different considerations of the suitability of overhangs.
Perhaps this is simply a comment a la "your results may vary," but here on the left coast I've found overhangs to be useful, but *not sufficient in passive solar design. Sun paths out here don't seem to quite align with our hottest temperatures-- in fact, currently in the Bay Area, we've been getting some of hottest temperatures of the summer over these past 2 weeks-- and the equinox was already last week! This is not uncommon for us here.

So, in one of the passive solar houses that I've been involved with, we're going through the exercise of installing some form of external window shading to supplement the overhangs, which are not terribly useful now (since they were designed for the solstice).

Exterior Shading can make up the difference--
Thus, I'm inclined to recommend operable exterior shading in addition to, or even in lieu of overhangs to the (rare) client who asks for a passive solar home (and even those who aren't since 8-12% south glazing is not uncommon here). With some proper training, it's amazing how exterior blinds can function to provide wonderful daylight (through indirect reflection), or allow for full sun through the windows (when they're fully retracted). It might take an extra minute for the occupant to operate the shades instead of simply operating a thermostat, but there are some that see the benefits are definitely worth their time. Exterior shades have their issues and expense, but on the positive side-- they're essentially a solar spigot that one can turn on or turn off at one's discretion.

As for the debate about Passivhaus vs. generic passive solar design--
Perhaps it's time to rehash some well articulated insights from Martin's post from a year ago:

Seasonal performance, Movable exterior shading
Actually Lucas, we have the same issues here. Typically, Passive Solar Designs with "proper" overhangs can become overheated in the fall and under heated in the spring. This is only true during the extreme weather of the season. Overhangs arent perfect but they work great considering the permanence.

I agree exterior movable shading devices can be a good fit depending on the situation. In my two identical homes example above, one homeowner would probably use them well. The other homeowner definitely would not. Its an occupant behavior factor that is tough to account for in a structure that could change ownership many times. For the most part, I dont think movable exterior shading is a good fit for cost-effective passive solar design. However, sticking with the bigger, fixed windows as advised would make them cheaper and quicker to adjust.

Monoslab on crawlspace/stemwall lot?
Thanks Kevin. I would much prefer a monoslab lot but its rare to find in our area and many others. Ive always considered a 2-3' elevation drop to be the point to switch to a stem wall which can be insulated just as well as a monoslab. Do you disagree?

Passive solar *is* greenwashing
This article omits a crucial component of passive solar design that renders it just another excuse for builders to greenwash their projects: There is no mention of moveable insulation. Without it, passive solar buildings barely break even, losing as much heat through the large windows at night and during cloudy weather as they gain on sunny days.

We have to expect more from our buildings now than the 10-30% reduction in green house gases passive solar allows, which means that any building on a site suitable for passive solar should be designed to use active solar now. Builders should be required to show that net solar gain makes up at least 50% of heat requirements and that solar gain does not add more than, say, 20% to cooling requirements. Except in very unusual situations passive solar will not get you anywhere near these goals and by competing with active solar components for space and budget, passive solar design will push a building even farther from them.

Response to Scott Raney
It sounds like all your passive solar experience is with poorly designed overglazed homes from the 70's. With 9%-12% glazing, high SHGC low e windows, tight envelope and good insulation, you just won't find any comfort problems.

Passive designs need thermal mass.
This was an excellent article, and could become the basis for "the bible" on passive solar designs.

One thing Brian failed to emphasize (in fact, he de-emphasized it) that would have resolved Scott Raney's issues, is the value of thermal mass. We have been building successful passive solar homes for a number of years. All of our passive solar homes have been on slabs, but our most recent designs even incorporate a slab on the second floor. This added thermal mass, contained entirely within the building envelope, is able to store enough energy during the day to fully heat the home during the night, with only one degree of temperature drop between the hottest temperature of the day, and the coldest of the night. Our most recent recordings were taken during a two-week period of 65 to 74 degree days, and 48 to 50 degree nights. Our inside temperatures stayed between 67 and 71 degrees during the entire period, with the maximum day-to-night swing at two degrees. This is without any heating or cooling system running.

I am expecting the day-to-night swings to increase during the winter, as the delta-t increases between the inside and outside over-night temperatures, but they will probably not exceed three or four degrees, based on our design degree day.

The amount of solar gain will vary widely on a day-to-day and season-to-season basis. Cloudy days will not yield much gain, whereas a sunny day, even in the winter, can fully condition your slab. The key is in having enough thermal mass, properly located, to store that heat until you need it.

Thermal mass is equally important in a hot climate. If solar gains are allowed to heat up air, at .018 btu per cubic foot, one square foot of window will raise the temperature of 10,000 cubic feet of air ten degrees in an average July day in my Zone 4 Marine climate. Imagine what a couple of hundred square feet of glass could do to the average 2,000 sf house, even with an SHGC of .30 or better! On the other hand, direct that radiant energy into a concrete floor, brick wall, or other appropriate thermal mass, and it will be almost completely absorbed, with only a couple of degrees of temperature rise.

Finally, don't underestimate the value of turning your "south" side to face twenty to thirty degrees east of south. This allows the home to warm up earlier in the morning, but the properly sized overhangs will shut the sun off sooner in the afternoon, to prevent overheating. Our Painted Hills house near Mitchell, Oregon (zone 5) has yet to turn on the heating or cooling system (min-split heat pump) since first occupancy last February.

Cost of upper Slabs and Thermal Mass benefit in humid climates
Thanks Ted. I am continually tempted to try the slabs on upper levels. This temptation is fueled by the many benefits of finished concrete floors more than the thermal mass properties. Iam curious if you have come up with any numbers associated with this increased cost. I know it gets tough when the concrete is to be the finished floor.

I read somewhere else that you use a 3" thickness. For screeds, Iam assuming you use double bottom plates of interior walls but what about the exterior wall SIPS? Any problems with cracks?

I think its easy for people on the West coast to forget about humidity issues when applying thermal mass to space conditioning needs in the East. I know it contributes to sensible loads but my feeling is that its not enough to justify much additional expense. The HERS models certainly dont give it much value.

Re: Passive designs need thermal mass
Ted Clifton I think has misunderstood my criticism, and indeed the entire physics of passive solar design. If you lose more heat through large uninsulated windows at night and on cloudy days than you gain on sunny days, no amount of thermal mass is going to make up for the fact that your "passive solar" house uses just as much energy to keep warm as a comparable house without "passive solar" features (i.e., the "passive solar" claim *is* just greenwashing). And without movable insulation, which this article doesn't address and the vast majority of modern "passive solar" designs don't include, net gain from the large windows passive solar design requires is small, perhaps even negligible. Certainly a small fraction of what net gain would be if active solar panels were used instead.
Thermal mass IMHO is most useful in cooling-dominated climates, especially in areas like high-desert where nighttime temperatures drop below indoor temperatures. In heating dominated climates thermal mass just makes the house slow to respond to changes in outdoor temps, compromising occupant comfort.

Misc. Passive Solar Article Comments
Brian has built one of our passive solar house designs and I am impressed to see his attention with the passive solar details and desire to write about it! Much of what he states is similar to what is addressed in my book "The Sun-Inspired House". With builders like this, we know that clients who have a passive solar house design will be able to achieve the intended goals of the design. The article is an excellent overview.

I like his statement "The most important detail of a passive solar design is an airtight, continuously insulated building envelope"; however, I might replace "passive solar design" with "low energy home" since I believe the most important element of a passive solar design is the south-facing glass since without that it would not be passive solar.

Regarding windows, I agree that it really a challenge for our clients (throughout North America) to obtain the high SHGC glass required for a passive solar home. A builder that is knowledgeable about passive solar window needs is rare, but a gem. We receive more calls about windows during construction of our sun-inspired designs than any other energy-related detail. I like the Canadian ER ratings of windows that take into account the SHGC in relation to the U value so we encourage looking for windows that have high ER ratings too (which is showing up more often in window spec sheets) for windows for the south wall. (We often design south windows to be of a different size than the north, west and east windows so that it is nearly impossible to have them installed on the wrong wall.

Regarding "A rule of thumb that works for most U.S. latitudes is that overhangs should be 12 inches above the window and 18 inches deep." While the 12" overhang edge above a window is a good rule, the 18" overhang length would be the rule for a 4' high window. A 24" overhang would be more the rule for a 6' tall window. Note "rule" as there are way too many exceptions.

Rather than extending overhangs, I often recommend a more easterly orientation which often can be up to 15 degrees east without overhang length substantially so am glad to see that Brian mentioned orientation other than due true south as an option. (With south walls oriented 45 degrees off of south, longer overhangs and shorter windows are often required so that orientation is advised only when small lots require that orientation.) The overhang design tool on http://www.susdesign.com is one that we have been using for years and Christopher Gronbeck's website is full of many other fun solar tools.

Regarding thermal and the reference to ICF especially, “Beware of advertised benefits from products that don’t have their thermal mass completely inside the insulated”, I too was initially concerned by this. After attending presentations where temperatures of homes with ICF walls were monitored, and after having clients living in climates throughout North America in ICF homes report the comfort and energy savings, I am happy to design with them if the client chooses and especially if the home owners are opposed to thermal mass in the floors.
With floor mass however, if a client desires the polished concrete floors or tile, the relatively new ICF floor systems have become one of my favorite forms of thermal mass for the main floor when the client also desires a basement. (If it is south-facing daylight basement we can design bedroom and living areas there to avoid the most costly, less-energy efficient location of those spaces otherwise on a second floor. ) Then we can leave the walls stick framed with blown cellulose and perhaps some additional foam insulation depending on the climate, stud wall thickness, etc. The ICF floors tie in very well with ICF basements walls that are typically used at least for the basement if not the main floor.

Adapting walls for a house design is fairly easy from an architectural standpoint. We often change an existing house plan to have ICF walls and if the design was already designed for a thicker 12” wood framed wall, the room sizes don’t change much and we can still keep the 2’ exterior dimensions that we like to design around as one of our sun-inspired design strategies to save on materials.

Wow, my few comments turned into a bit more. Thanks again Brian for the inspiration to elaborate on passive solar. I am passionate about this low-tech use of the sun's energy.

Thanks Debra!
Excellent points. Loved the tip about different sizing of windows to avoid misplacement on specific orientations. For anyone searching for existing passive solar designs or a custom designer, Debra and http://www.sunplans.com/ is one of the best resources available.

Response to Michael Scott
Michael,
There are several issues here:
1. If below-grade space is properly insulated and waterproofed, it costs more than above-grade space.

2. Bedrooms require emergency egress windows with window sills (stools) at a specified height so that people (including the elderly) can escape in an emergency.

3. Once you have installed adequate insulation in your walls, the advantage of below-grade soil temperatures either disappears or becomes much less significant than the advantage would be for an uninsulated building.

"Ultimate" Passive Solar
Thanks for a wonderful, broad-reaching, passive residential construction 'primer', Brian.
Most, if not ALL of your comments and recommendations are logical to the point of almost being self-evident, but it takes a real knack to put them into language that both builders and users (buyers) can understand.
I have just one BIG question that I'm hoping you'll answer, and that is: why is it that so many of the plans, designs, proposals I see for passive home building do not include partial earth sheltering in the form of a "foundation"....or "basement"?
Digging a hole in the ground and pouring a cement "liner" is, ironically, the first step of building a conventional (non-solar) home in most of Central Canada (Ontario)....indeed, much of our country. And, although I understand that in many parts of North America pouring a foundation is not standard practice, would it not be one of the smartest, most effective ways to begin the design/construction of an efficient passive solar home?
With intelligent "green" insulation and waterproofing measures (some of which are already incorporated into "normal" homes with basements, shouldn't the use of a mostly-underground lower living level be a good way to minimize heating and cooling requirements based on half of the dwelling being geo-thermally protected? Granted, below-grade living quarters DO still have a bit of rat/mole stigma attached to them.... but surely the solar community is beyond this sort of stereotyping now with all the amazing subterranean luxury homes that have graced the pages of innumerable "home of the future" web and paper publications.
To be blunt, I just don't have any idea how the "true cost" of starting with a foundation compares to the price of building an identical (in sq feet) all-above-ground, two-storey house .
Do you, or any others have experience in pricing out these two methodologies?
NOTE: The "basement" model I would use for comparison would have identical window area by virtue of being a "raised bungalow" with "pill-box" style windows placed high in the foundation walls. The lower level would be utilized for the same sorts of activities as the North-facing rooms of "normal" solar homes: sleeping, storage, utilities, etc.
Thanks for any insights/comments.

Thanks, Martin!
Any ONE of your reasons would seem to be ample; the three COMBINED certainly make it a no-brainer!

The only remaining 'challenge', I guess, would be from a resale standpoint, if it DOESN'T have a basement (at least in Ontario), there seems to typically be (even in my OWN mind) and bit of a "let down" when the buyer discovers a basement is "missing".

I guess that's all a matter of mind-set and could be easily offset with a well integrated storage/workroom area on the main floor.

Functionality in one tool
Well, if you don't mind I would like to recommend you a tool that I've came across recently. It's called easysolar (just visit http:easysolar.co). It's an app available on smart phones (and lps too) that facilitates the design of pv panels. including the measurement of angles, azimuth, shadiness, takes into account the seasonal changes and it's just in one small cell. Certainly, some of you will find it extremely useful.

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